The present invention relates generally to Code-Division Multiple Access (CDMA) communication systems and other types of multiaccess systems which utilize orthogonal data modulation, and more particularly to signal detection techniques for use in such systems.
The IS-95 CDMA standard specifies techniques for implementing cellular and PCS-band mobile wireless communications. Designed for handling voice calls, IS-95 systems provide a significant capacity increase over previous wireless systems. A conventional base station detector configured in accordance with the IS-95 standard uses a combination of selection diversity (SD) for choosing the strongest signal among the signals of several receive antennas and equal gain combining (EGC) for summing the energies of these signals. The conventional base station detector also uses noncoherent energy detection for deciding among the available orthogonally-modulated CDMA signals for each user. Additional details regarding IS-95 systems can be found in, for example, TIA/EIA/IS-95A, xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d June 1996, which is incorporated by reference herein.
FIG. 1 shows a conventional IS-95 noncoherent base station detector 10 for detecting an uplink orthogonally-modulated CDMA signal transmitted by a kth user. The detector 10 includes A receive antennas 12-a, a=1, 2, . . . A. Each of the antennas 12-a is coupled to a corresponding downconverter 14-a. The downconverted baseband received signal from the ath antenna is designated ra(t). Assuming that the multipath delays are known perfectly, for a given resolvable multipath component with a given delay, the received signal at the ath antenna is correlated with M data signals sk(m)(txe2x88x92xcfx84k, l), where a specifies one of the A antennas, l=1, 2, . . . L specifies one of the multipath components of the kth user, and m=1, 2, . . . M specifies one of M possible data symbols. Each correlator in the detector 10 includes a multiplier 16(m)xe2x88x92l, a, a matched filter 18(m)xe2x88x92l, a and a squaring device 20(m)xe2x88x92l, a. The detector 10 uses selection diversity of order L (i.e., SD(L)) among the multiple receive antennas to pick out the strongest L signals for further processing. This selection process is implemented in selection devices 22(1), 22(2), . . . 22(M). The outputs of each of the selection devices 22(m) are summed in a corresponding summing device 24(m). For each of the M data symbols, the resulting largest L correlator outputs as determined by selection device 22(m) are combined using equal gain combining (i.e., EGC(L)) in summing devices 24(m) to form a decision statistic zk(m). A symbol estimate {circumflex over (m)}k for the kth user is then generated by selecting the largest of the M decision statistics zk(m) in a selection device 26.
A number of techniques have been proposed to improve the performance of the conventional detector of FIG. 1. One such proposed technique utilizes a reverse link pilot signal transmitted by each mobile user to aid in channel estimation. However, the use of such a signal would require changes to the IS-95 standard and hence to system hardware and software configured to operate in accordance with that standard. Another proposed technique utilizes an adaptive antenna array (AAA) receiver in which antenna weights are adaptively adjusted to account for MAI. However, this approach requires significant additional computational power to adaptively adjust the antenna weights. These and other techniques have therefore been unable to significantly increase system capacity, or equivalently, reduce signal-to-interference requirements, without introducing inefficiency or requiring substantial changes in the IS-95 standard.
We have discovered that detection of orthogonally-modulated signals can be considerably improved through the use of coherent channel estimates. Advantageously, the use of coherent channel estimates in accordance with the invention allows the implementation of performance improvement techniques such as maximal ratio combining (MRC), coherent detection and interference cancellation. In an illustrative embodiment, a matched filter bank provides a set of matched filter outputs for different combinations of data symbols, received signal multipath components, and base station receive antennas. The matched filter outputs are processed in a coherent channel estimator to generate a set of coherent channel estimates. The channel estimates may be generated directly from the matched filter outputs, or may be generated by processing the matched filter outputs using a decorrelating detector approach which involves multiplication by a decorrelation matrix.
An exemplary coherent channel estimator in accordance with the invention receives a set of matched filter outputs. These outputs for a kth user, k=1, 2, . . . K, are represented as y(m)k, l, a, where a=1, 2, . . . A specifies one of A receive antennas, l=1, 2, . . . L specifies one of L multipath components of the kth user, and m=1, 2, . . . M specifies one of M data symbols. The coherent channel estimator for antenna a includes a symbol-driven selection device associated with the kth user that receives as its input ML matched filter outputs y(m)k, l, a for the kth user. The selection device is operative to select, for the kth user, L of the ML matched filter outputs which correspond to an estimate of a selected symbol for that user from a previous symbol interval.
The coherent channel estimator further includes L buffers for the kth user, with each of the buffers having a width W corresponding to a designated number of symbol intervals, for storing the L selected matched filter outputs as received from the selection device. The L buffers and a selection device may be provided for each of K users, in conjunction with L summing devices for each of the K users. Each of the L summing devices for a given user is operative to sum the matched filter outputs from a corresponding one of the buffers, to form a vector. The coherent channel estimator also includes a multiplier operative to multiply the vector by a designated quantity to obtain the corresponding channel estimates. This designated quantity may include, for example, a factor 1/(W{square root over (Es+L )}), where W is the width of the buffer and Es is the symbol energy. In an embodiment which utilizes the above-noted decorrelating detector approach, the designated quantity may also include a decorrelation matrix generated for the vector.
In accordance with another aspect of the invention, the above-noted interference cancellation may be implemented for a kth user as a multistage process. A first stage of the process generates preliminary symbol estimates for each of a plurality of interfering users j, j=1, 2, . . . K, jxe2x89xa0k, of the system. A second stage reconstructs received signals for the interfering users, utilizing (i) the preliminary symbol estimates for those users, (ii) information regarding spreading-symbol codes and delays associated with those users, and (iii) the channel estimates.
The second stage also subtracts, from a stored copy of the received signal, multiaccess interference attributable to the interfering users, so as to form an enhanced received signal for the kth user. A third stage processes the enhanced received signal to generate a symbol estimate for the kth user. These symbol estimates can be used in place of the preliminary symbol estimates in the second stage to drive another round of interference cancellation. Furthermore, such iterations can continue indefinitely using increasingly refined symbol estimates. In order to reduce detection complexity, regardless of the number of iterations, these interference cancellation techniques may be provided for only a subset of a plurality of users of the system. Moreover, interference cancellation in accordance with the invention may make use of coherent channel estimates generated in the manner described above, or coherent channel estimates generated in any other suitable manner.
A signal detector in accordance with the invention can provide significantly improved performance relative to a conventional noncoherent signal detector. For example, generating coherent multipath channel estimates for each of a number of receive antennas permits the use of MRC, which can improve performance by several decibels relative to the conventional detector. In addition, the relative gains associated with MRC increase as the number of antennas increase. Further improvement could be provided by utilizing coherent detection. In an IS-95 system with 64-ary orthogonal modulation used in the IS-95 system, the gain from use of noncoherent detection will be about 1.5 dB. Still further gains can be obtained through the use of the above-described interference cancellation.